correct initialization code (#3376)

Explicitly initialize struct members rather than relying on compiler extensions.
diff --git a/Python/pylifecycle.c b/Python/pylifecycle.c
index f33e920..3f405b1 100644
--- a/Python/pylifecycle.c
+++ b/Python/pylifecycle.c
@@ -77,7 +77,7 @@
 extern void _PyGILState_Fini(void);
 #endif /* WITH_THREAD */
 
-_PyRuntimeState _PyRuntime = {};
+_PyRuntimeState _PyRuntime = {0, 0};
 
 void
 _PyRuntime_Initialize(void)